1. Field of the Invention
The present invention relates to a process for the manufacture of light granular materials from silico-aluminous substances such as shales, clay, ashes or certain industrial residues. These materials are suitable for use in building or in various other fields, such as the manufacture of light concretes, or thermal insulation.
2. Description of the Prior Art
Up to now the manufacture of artificial light granules has necessitated the preparation of a crude product in the form of nodules, rods, pellets or any other type of agglomerate in lump form of various sizes, the mechanical strength of which after drying had to be sufficient in order to avoid any appreciable subsequent dimensional breakdown during handling prior to firing. The lightening of the dry agglomerated product was obtained by expansion at a temperature at which the material was brought to melting point and at which the gases formed as a result of complex reactions inside the mineral material caused the swelling of the softened granule.
In the process of the prior art, the gases causing the expansion arise from oxidation/reduction and dissociation reactions inside the silico-aluminous material and possibly from the decomposition of porogenic agents added to the raw material. This evolution of gas must be produced at the moment when the material has a suitable plasticity so that the fine bubbles of gas can expand sufficiently without however escaping from the granule.
The fact that the evolution of the gases takes place simultaneously with a certain pyroplastic state of the material permitting the swelling, limits the number of silico-aluminous products which can be used to obtain lightened products. Of course, it is possible to correct the properties of the raw material by the addition of fluxing products (lime, dolomite, alkali salts, iron oxide) or porogenic products, but at the cost of complication of the processing installations used and an increase in the cost price.
On the other hand, the necessity of passing through a softened phase favours the sticking of the grains to one another or on to the walls of the treatment apparatus.
Finally, the nature of the process makes it necessary to employ raw materials which are poor in carbon, which sometimes means, for example in the case of coal shale, a prior "decarbonation" treatment.